The field of the invention is nuclear magnetic resonance (NMR) methods and systems. More particularly, the invention relates to the compensation of NMR systems for variations in the polarizing magnetic field.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. An NMR signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, and this signal may be received and processed.
When utilizing these NMR signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It is required that the polarizing field B.sub.0 be stable during the series of imaging sequences, which may have a duration of a few seconds to fifteen minutes. The required stability is quite high, and typically changes of from 0.1 to a few parts per million (ppm) can degrade the spectra or image. The stability requirement is thus quite severe, and is especially difficult to achieve in resistive and permanent magnets, as opposed to superconducting magnets.
The instabilities may be caused by external disturbances, (e.g. moving ferrous masses such as elevators), and imperfections in the magnet system. In resistive magnets such imperfections include instabilities in the magnet current and thermal contraction of the coils. In permanent magnets the ambient temperature affects both the dimensions of the magnet and the flux produced by its material and thus this type of magnet is quite sensitive to thermal fluctuations.
The polarizing field stability requirement has been addressed in U.S. Pat. No. 4,623,843 of Macovski, and U.S. Pat. No. 4,417,209 of Hounsfield. They teach how to measure the presence of unwanted fluctuations in the polarizing field .DELTA.B.sub.0 and how to use the measured signal to compensate for .DELTA.B.sub.0. This .DELTA.B.sub.0 measurement is done by NMR, using a separate reference sample located outside the imaged object but inside the B.sub.0 field region. The reference sample can be excited separately or by the same RF excitation field as that of the object. The reference signal from the sample produces an NMR signal having a frequency f.sub.0 +.DELTA.f.sub.0, which is proportional to B.sub.0 +.DELTA.B.sub.0. The measured frequency changes .DELTA.f.sub.0 are used for demodulating the acquired NMR signals received from the object, thus compensating for the .DELTA.B.sub.0 instabilities during reception. Methods for compensating the magnitude of the polarizing field B.sub.0 during excitation of the object are also described.
Prior systems for compensating .DELTA.B.sub.0 have several basic flaws. The gradients used during the spectroscopic or imaging sequence cause additional magnetic fields at the points where B.sub.0 is being measured, and they change very rapidly with time. This fact has made it impossible to use a continuously measuring NMR-probe. The NMR B.sub.0 sensors, therefore, have to be used in the pulsed mode, which means that the compensation signal is available only during part of the scan time. This complicates its use because it has to be coordinated with the imaging pulse sequence to provide signal when needed.
Another problem with the magnetic field gradients is that they decrease the obtainable NMR signal from the sensor, because they dephase the NMR signal across its sensitive volume. This results in a shortening of the duration of the nuclear free precession signal obtained for each pulse. This means that the sensitive volume of the sensor cannot be very large. As a consequence the signal-to-noise (S/N) ratio of the NMR sensor is at best only a little better than needed and at worst it can degrade the final image quality.
As disclosed in U.S. Pat. No. 5,488,950, one solution to these problems is to employ electron-spin resonance (ESR) as a means for measuring changes in the polarizing magnetic field. As with NMR, ESR employs a strong polarizing magnetic field to measure resonant signals, but in ESR the resonant signals are produced by electrons rather than nuclei. ESR typically operates at microwave frequencies and employs electronics and coil structures that are substantially different than those employed in NMR.
In recent years small rf coils called "microcoils" have been developed for use in NMR spectroscopy and NMR imaging. As described in U.S. Pat. Nos. 5,684,401 and 5,654,636, microcoils are used in spectroscopy to obtain NMR signals from very small samples. As disclosed in U.S. Pat. No. 5,655,234, microcoils are also used in NMR imaging applications where they are imbedded in medical instruments (e.g. catheters, biopsy needles, etc.) and used to produce signals from which their location in the patient can be determined.